Sky & Telescope - March 2019 - 12

COSMIC RELIEF by David Grinspoon

Sudden
Impact

79°N

The discovery of a recent huge
meteor crater beneath Greenland's ice sheet has reignited a
debate over cosmic inﬂuences.
750 m

IN THE 1960s, we took a giant leap for-

M A RCH 2 019 * SK Y & TELESCOPE

0m
Height
above
ellipsoid

78°30′N

Kilometers

66°W

call deep time. Slow changes acting
over previously unimagined temporal
expanses - millions of years or more -
could raise mountain ranges and carve
deep canyons.
Yet the evidence for impacts indicated that some important geologic
changes are indeed sudden and
catastrophic. The new catastrophists
sometimes went a bit overboard, however, attempting to explain everything
mysterious with large impacts. At worst,
meteor strikes became a kind of deus ex
machina, an explanation to invoke when
evidence was lacking.
One example of this, arguably, was
an attempt to assign blame to an impact
for the Younger Dryas cooling episode
around 12,000 years ago. This period
roughly correlates with the extinction
of many large mammal species and the
possible disappearance of the Clovis
people. Critics disputed the evidence,
pointing out that no crater of the right
age had turned up. The debates over this
became quite heated and, regrettably,
sometimes acrimonious.
The recent discovery in northwest
Greenland of a large impact crater,
31 km across and possibly as young as
12,000 years, has predictably rekindled

64°W

this controversy, with proponents of
a Younger Dryas impact feeling vindicated and detractors digging in.
Certainly, it's a captivating possibility. If a crater that massive really
is so young, then one would expect
demonstrable effects on human and
natural history at that time, perhaps
matching the alterations that occurred
then in North America. But the age of
the impact is not precisely known. The
12,000-year number is a lower limit for
the age; it could be much older.
The best and perhaps only way to
determine when the impact took place
will be to drill down through the area's
nearly kilometer-thick ice, retrieve
samples of melted rock from the crater
itself, and determine their ages in a
laboratory using radiometric isotopes.
It will be some time before this
can happen. Until we know when this
impact occurred, the smart attitude is
to reserve judgment. Meantime, it will
be fascinating to watch the debate play
out - from a safe distance.
■ DAVID GRINSPOON is an astrobiologist who, for his PhD dissertation, modeled the effects of large impact events
on the evolution of Earth-like planets.

K . H. KJÆR E T A L. / SCIENCE ADVANCES 2018, CC BY-NC 4.0

ward in our understanding of the threat
that meteors pose to our planet. In
1960, Eugene Shoemaker ﬁrmly established that a meteor - and not a volcanic eruption or other cause - created
Arizona's Meteor Crater. A huge explosion by an iron meteorite slamming
into Earth 50,000 years ago gouged
out the 1.2-kilometer-wide pit. This
conﬁrmation, combined with detailed
lunar photography that deﬁnitively
showed that the Moon's craters arose
from impacts, represented an important
watershed in our comprehension of
the profound connections between our
planet and the rest of the solar system.
The father-and-son team of Luis
and Walter Alvarez starkly illustrated
these connections in 1980. That's the
year they convincingly argued that
an asteroid impact 65 million years
ago triggered the mass extinction that
wiped out the dinosaurs (and most
other species). We realized then that
occasional, monstrously violent celestial
bombardments have repeatedly inﬂuenced biological evolution on Earth.
This ushered in what some called the
"new catastrophism," whose exponents
proposed impact events as the cause
of many an unexplained and climactic change in our planet's history.
The phrase is a nod to the old tension
within geology between catastrophism and uniformitarianism. In the 19th
century, it was a breakthrough when
we realized that colossal transitions
in Earth history did not require catastrophic happenings, just what we now

Sky & Telescope - March 2019

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